Pipes (having a circular section) having good rigidity and good collision strength are used for some automative parts. Also, many parts having a varying diameter are used from the viewpoint of joining to other parts. As a manufacturing process to obtain pipes with different diameters along a longitudinal direction, there are methods in which metal pipes manufactured by a process such as a UOE process or roll forming are subjected to secondary processing for pipes such as reducing, flaring, or hydroforming (these methods are referred to as “related art I”). There also is related-art regarding a method of manufacturing the pipe with different diameters along a longitudinal direction formed by press forming. According to this known forming method (see Japanese Patent No. 4713471; referred to as “related art II”), to suppress defects such as wrinkling and reduction of the sheet thickness after forming, the shape of the blank is improved and O-shape forming is performed after a U-shape forming has been performed.
With the related art I, pipes having been formed are subjected to secondary processing such as reducing, flaring, or tube forming. Thus, dedicated processing apparatuses are required. This may reduce productivity and lead to an increase in the cost. Furthermore, reducing and flaring are performed on limited positions in many cases, that is, mainly on positions near ends of pipes. Thus, there is a problem with versatility in these methods. With tube hydroforming, the sectional shape can be arbitrarily changed in the longitudinal direction. However, the sheet thickness of a protruding portion is significantly reduced. Thus, it is difficult to obtain a component having a uniform thickness. In addition, since the time required to form is long, there is a problem with productivity.
In the related art II, vertical wall portions of a U-shaped formed part are inserted into an upper die during O-shape forming. This requires a core referred to as a guide blade. A step in which end portions of the blank are bent inwardly is also required before U-shape forming is performed. Furthermore, in the related art I, accuracy in sectional dimensions in a formed product is not described. Accuracy in sectional dimensions is important when a formed product is used as an automative part from the viewpoint of the performance of the part such as rigidity and for assembly. That is, there are problems with the manufacturing cost and the accuracy in product dimensions in the related art II.
That is, with the related art, there is a problem in that a pipe with different diameters along a longitudinal direction, which is manufactured at high productivity and a low cost and with good accuracy in product dimensions, cannot be provided.